My Photo Stream not sharing to MBP

Any ideas why My Photo Stream has suddenly stopped sharing with my MBP? All was well until a few days ago (March 2) when a series of iPhone pictures stopped syncing halfway through, and no others have appeared since. They ARE on my iPad, however.

There are currently 521 photos on My Photo Stream on my phone and iPad (both 9.2.1) but only 278 on the MBP (10.11.3). As they are supposed to sync (when wifi is on), how is this?

My Photo Steam is 'ON' with all three devices. Have switched wifi on/off on all devices, no change.

Any suggestions appreciated.

Well, it took a factory reset of the iPhone in the end. Talked through back up and restore by Apple Support who identified it as an issue with the phone rather than the MBP.

Found a procedure online which involved Utilities > Activity Monitor > Photos Agent > Force Quit which worked but had the same problem the following day.